The Synchronous Digital Hierarchy (SDH) is a very widely used transmission system for the transmission of a large number of telephone channels between nodes in a transmission network. The system permits transfer of asynchronous signals, such as e.g. signals from a Plesiochronous Digital Hierarchy (PDH) of 140, 34 and 2 Mbits/s, said signals being inserted or mapped into the SDH system. Standards prescribe how to place the individual signals in the individual bytes in the pulse frame of the SDH system.
Frequently, it is desirable to be able to transmit a user channel together the data proper (i.e. the telephone channels). It may e.g. be a monitoring channel for the equipment which is connected to a 2 Mbits/s section, it being desirable to transmit the channel together with the associated 2 Mbits/s signals. So far, this has just been possible by using overhead bytes, as e.g. the overhead bytes called D1-D12 in the SDH system may be used in certain situations for transferring user channels. However, these can just be used for transferring data between two adjacent elements in the network, as each element decides for itself what it wants to use them for, and in certain situations they are reserved in advance for other purposes. It is thus not possible to use these bytes when other network elements are passed en route. Furthermore, the capacity of these bytes is rather limited, so that only small amounts of data can be transferred in this way.
However, when the PDH signals are inserted into the pulse frame of the SDH system, they will not use all the bytes of the pulse frame because of the absent synchronization of the two systems. Thus, there is a plurality of empty bytes which are merely used as a filler between the bytes which contain PDH signals.